1. Fields of the invention
The present invention relates to a window shade, and more particularly, to a transmission device having multiple roller units to respectively control the mediate bar and the bottom bar of the window shade.
2. Descriptions of Related Art
The conventional cordless window shade known to applicant is disclosed in U.S. Pat. No. 7,487,817, and comprises transmission cords of the transmission device and the movable unit and the fixed unit, wherein the operation cords are wrapped to the movable unit and the fixed unit. The transmission cords are cooperated with the operation cords so that the users simply push or pull the bottom bar of the window shade to change the wrapping status of the operation cords relative to the movable unit. The operation cords and the transmission cords are cooperated with each other to achieve a balance, such that the users can lift or expand the window shade by pulling or pushing the bottom bar.
U.S. Pat. No. 9,316,050 uses a single spring to replace the controller of the movable unit disclosed in the U.S. Pat. No. 7,487,817. The spring is connected between the top box and the movable unit.
Applicant has invented the window shade with a top box, a mediate bar and a bottom bar. The top box includes a cordless device on each of two ends thereof, and the cordless devices comprise even number of transmission cords to respectively be connected to the mediate bar and the bottom bar. The mediate bar can be individually moved to form an area that does not have any slats of the shade. Therefore, the users can see the outside scene via the area. This specific arrangement is disclosed in US Patent Application Publication No. 2015/0041073.
However, when the window shade has a certain length, the transmission cords have to be long enough, and the top box may not have sufficient space to accommodate the transmission cords when two sets of the scrolling units, two movable units and two springs are located in the top box.
The present invention intends to provide a transmission device of a window shade, wherein the scrolling units and the movable units are improved so as to be installed in the top box while the transmission cords of longer length can still be received in the top box. The operation to the cords that are connected to the mediate bar and the bottom bar is smooth.